The striker fired in his 24th goal of the season just four minutes after the interval to cancel out Alex Revell's first-half opener for Steve Evans' side.
Garner's effort was of the highest quality, the 26-year-old taking two touches without the ball bouncing before arrowing a wonderful effort into the top corner from 20 yards.
It is the third time that the two sides have drawn this season, with February's meeting at Deepdale ending 3-3 and the reverse fixture finishing goalless.
Britt Assombalonga's 33rd goal of the season in all competitions was eclipsed by Moses Odubajo's second-half equaliser as Leyton Orient claimed a 1-1 draw at Peterborough United in the other semi.
Former Southend United striker Assombalonga put Darren Ferguson's side on course for a second Wembley visit of the season, with the club having won the Football League Trophy final last month.
But Orient scored a deserved equaliser as a sweeping move was capped by Odubajo's tap-in after Robert Olejnik had parried David Mooney's effort.
